This invention relates to radiographic imaging techniques in which an extended source of spatially coded X or gamma radiation is utilized for illuminating a subject to produce a coded image thereof with an image of the subject being obtained by a decoding of the coded image.
Radiographic imaging utilizing coded sources is taught in the U.S. Pat. No. 3,748,470 which issued on July 24, 1973 in the name of H. H. Barrett wherein it is shown that imaging of a subject can be obtained by a coded source in combination with a mask that produces spatial coding of the X or gamma radiation whether the masking be done at a distance from the source or directly on the source itself so that, in the latter case, the source distribution of radioactive substances is itself a spatially modulated pattern of radiation. Therein, Barrett discloses a chirped checkerboard pattern with two dimensional matched filtering being utilized for decoding the detected coded or scrambled image to produce a true representation of the subject. The use of a Fresnel pattern in lieu of the chirped checkerboard is taught in U.S. Pat. No. 3,936,639 which issued Feb. 6, 1976 in the name of H. H. Barrett with optical reconstruction being utilized to form the desired image of the subject. With respect to the Fresnel pattern, it is noted that an on-axis pattern or, preferably, an off-axis Fresnel pattern may be utilized. In the case of the off-axis Fresnel pattern, reduction of noise is attained in the resulting image with the reconstruction process utilizing viewing optics angled with reference to a normal of a transparency of the coded or scrambled image. These reconstruction procedures are applicable, in view of the aforementioned teaching of Barrett in U.S. Pat. No. 3,748,470, whether the coding be accomplished by a mask interposed between the subject and the detector or by a spatial coding on the source itself since the coded image has the same form in either case.
A problem arises in that the small thickness of blood vessels being imaged results in absorption of radiation much less than that absorbed by the total thickness of the body. This detracts from the desired image when, as in angiography, it is desired to produce an image only of a specific element, typically the iodine of the contrast agent which is injected into the blood vessels of the subject to be imaged.